Notch signaling is a highly evolutionarily conserved pathway implicated in diverse functions including stem cell maintenance, cell fate specification, cell proliferation, and apoptosis. When membrane-bound Notch receptors recognize ligands of the Delta and Jagged families, they are cleaved by metalloproteases and the γ-secretase complex, allowing the release of the intracellular domain into the nucleus where it associates with co-factors to control a significant number of targets including the Hes family of genes (Artavanis-Tsakonas et al., “Notch Signaling: Cell Fate Control and Signal Integration in Development,” Science 284:770-776 (1999); Ilagan and Kopan, “SnapShot: Notch Signaling Pathway,” Cell 128:1246 (2007)). In the hematopoietic system, Notch is essential for the emergence of definitive hematopoietic stem cells (HSC) during fetal life (Robert-Moreno et al., “Impaired Embryonic Haematopoiesis Yet Normal Arterial Development in the Absence of the Notch Ligand Jagged1,” Embo J. 27:1886-1895 (2008)) and indispensable for the commitment of progenitors to the T cell lineage (Zuniga-Pflucker, J. C. “T-cell Development Made Simple,” Nat. Rev. Immunol. 4:67-72 (2004)). Moreover, Notch1 appears to be the central oncogenic trigger in T cell acute lymphoblastic leukemia (T-ALL) in both humans and mice (Weng et al., “Activating Mutations of NOTCH1 in Human T Cell Acute Lymphoblastic Leukemia,” Science 306:269-271 (2004)). Indeed, Notch1 (or its regulator Fbw7) are commonly mutated leading to constitutive activation of the Notch pathway in the majority of T-ALL patients (Malyukova et al., “The Tumor Suppressor Gene hCDC4 is Frequently Mutated in Human T-Cell Acute Lymphoblastic Leukemia With Functional Consequences for Notch Signaling,” Cancer Res. 67:5611-5616 (2007); Maser et al., “Chromosomally Unstable Mouse Tumours Have Genomic Alterations Similar to Diverse Human Cancers,” Nature 447:966-971 (2007); Thompson et al., “The SCFFBW7 Ubiquitin Ligase Complex as a Tumor Suppressor in T Cell Leukemia,” J. Exp. Med. 204:1825-1835 (2007)). In contrast to the T cell lineage where the role of Notch signaling is well defined, there is conflicting information on the role of Notch signaling in the function of adult stem cells (HSC), multipotential progenitors (MPP) and in the myelo-erythroid compartment (Dahlberg et al., “Ex vivo Expansion of Human Hematopoietic Stem and Progenitor Cells,” Blood 117:6083-6090 (2011); Delaney et al., “Notch-Mediated Expansion of Human Cord Blood Progenitor Cells Capable of Rapid Myeloid Reconstitution,” Nat. Med. 16:232-236 (2010); Maillard et al., “Canonical Notch Signaling is Dispensable for the Maintenance of Adult Hematopoietic Stem Cells,” Cell Stem Cell 2:356-366 (2008)). Initial in vitro reports suggested that Notch signaling accelerates myeloid differentiation (Schroeder et al., “Notch Signaling Induces Multilineage Myeloid Differentiation and Up-Regulates PU.1 Expression,” J. Immunol. 170:5538-5548 (2003); Tan-Pertel et al., “Notch Signaling Enhances Survival and Alters Differentiation of 32D Myeloblasts,” J. Immunol. 165:4428-4436 (2000)). However, subsequent studies contested this conclusion. Most notably, it was shown that Notch can suppress myelopoiesis in vitro (de Pooter et al., “Notch Signaling Requires GATA-2 to Inhibit Myelopoiesis From Embryonic Stem Cells and Primary Hemopoietic Progenitors,” J. Immunol. 176:5267-5275 (2006)), and Gilliland and colleagues reported that Notch signaling can induce megakaryocyte differentiation (Mercher et al., “Notch Signaling Specifies Megakaryocyte Development From Hematopoietic Stem Cells,” Cell Stem Cell 3:314-326 (2008)). It has recently been shown that Notch signaling can function as an antagonist of the granulo-monocytic progenitor (GMP) cell fate and that loss of Notch signaling biases commitment towards GMP differentiation, eventually resulting in chronic myelomonocytic leukemia (CMML) (Klinakis et al., “A Novel Tumour-Suppressor Function for the Notch Pathway in Myeloid Leukaemia,” Nature 473:230-233 (2011)), a myelodysplastic/myeloproliferative overlap syndrome. Inactivating mutations in the Notch pathway were also observed in a fraction of CMML patients, suggesting that this pathway is targeted by genetic alterations. These data are consistent with subsequent reports of inactivating Notch pathway mutations in head and neck cancer (Agrawal et al., “Exome Sequencing of Head and Neck Squamous Cell Carcinoma Reveals Inactivating Mutations in NOTCH1,” Science 333:1154-1157 (2011); Stransky et al., “The Mutational Landscape of Head and Neck Squamous Cell Carcinoma,” Science 333:1157-1160 (2011)). However, none of these studies were able to prove that Notch could function as a tumor suppressor in vivo. For example the data was not able to prove direct involvement of Notch signaling in myeloid disease, as Notch deletion did not lead to transplantable frank myeloid leukemia. These studies also did not test whether Notch pathway activation can target established disease, something of unique clinical significance.
Acute Myeloid Leukemia (AML) is a clonal hematopoietic neoplasm characterized by the proliferation and accumulation of myeloid progenitor cells in bone marrow, and is the most common acute leukemia diagnosed in adults. Outcomes for AML patients remain poor, despite the use of cytotoxic chemotherapy and stem cell transplantation most patients die of relapsed, refractory AML (Frohling et al., “Genetics of Myeloid Malignancies: Pathogenetic and Clinical Implications,” J. Clin. Oncol. 23:6285-6295 (2005)). Cytogenetic and molecular studies have shown that AML is a heterogeneous disease in which a variety of cytogenetic and molecular alterations have biologic and clinical relevance (Armstrong et al., “MLL-Rearranged Leukemias: Insights From Gene Expression Profiling,” Semin. Hematol. 40:268-273 (2003); Dash and Gilliland, “Molecular Genetics of Acute Myeloid Leukaemia,” Best Pract. Res. Clin. Haematol. 14:49-64 (2001); Dohner et al., “Diagnosis and Management of Acute Myeloid Leukemia in Adults: Recommendations From an International Expert Panel, on Behalf of the European LeukemiaNet,” Blood 115:453-474 (2010)). These include chromosomal abnormalities, which lead to generation of leukemogenic fusion oncoproteins, including Mixed Lineage Leukemia (MLL) gene fusions which are associated with adverse outcome. In addition, somatic mutations in tumor suppressors have been shown to contribute to leukemogenesis and improve AML risk classification (Bacher et al., “Molecular Genetics in Acute Myeloid Leukemia,” Curr. Opin. Oncol. 22:646-655 (2010)). However, molecular mechanisms linking these mutations to transformation are incompletely understood, and the role of the most recently identified genes, including TET2, ASXL1 and IDH1/2 in AML pathogenesis has not been fully delineated. Current treatments for AML patients include dose-intensive chemotherapy and stem cell transplantation, which are associated with significant toxicities and high relapse rates. Thus, identification of new signaling pathways of which activation or inhibition will lead to therapeutic targeting of AML cells is of urgent clinical significance